The mechanism of the phosphorylation-dependent myosin-linked regulation of vertebrate smooth muscle is being investigated. To study this, we prepared myosin and heavy meromyosin from turkey gizzards. We find that the two subunits (or heads) of a myosin molecule interact during regulation and that formation of filaments further affects this head-head interaction. We are trying to investigate the function and the substructure of myosin using controlled proteolytic digestion and are trying to prepare active subfragments from human platelet myosin with hopes of studying its mechanisms of regulation.